18 years old Kang Kyung Joon (Shin Won Ho)’s spirit suddenly becomes transplanted in the body of 30 years old Seo Yoon Jae (Gong Yoo). Seo Yoon Jae is a successful doctor and engaged to high school teacher Gil Da Ran (Lee Min Jung). Gil Da Ran just so happens to be the teacher of 18 years old Kang Kyung Joon.